Changes: Immunoglobulin G

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IgG is the most abundant immunoglobulin and is approximately equally distributed in blood and in tissue liquids, constituting 75% of serum immunoglobulins in humans.[1] IgG molecules are synthesized and secreted by plasma B cells.

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IgG antibodies are predominantly involved in the secondary immune response (the main antibody involved in primary response is IgM). The presence of specific IgG generally corresponds to maturation of the antibody response.[2]

IgG antibodies are large molecules of about 150 kDa composed of 4 peptide chains. It contains 2 identical heavy chains of about 50 kDa and 2 identical light chains of about 25 kDa, thus tetrameric quaternary structure. The two heavy chains are linked to each other and to a light chain each by disulfide bonds. The resulting tetramer has two identical halves which together form the Y-like shape. Each end of the fork contains an identical antigen binding site. The Fc regions of IgGs bear a highly conserved N-glycosylation site. The N-glycans attached to this site are predominantly core-fucosylated diantennary structures of the complex type. Additionally, small amounts of these N-glycans also bear bisecting GlcNAc and α-2,6 linked sialic acids residues.[3]

Note: IgG affinity to Fc receptors on phagocytic cells is specific to individual species from which the antibody comes as well as the class. The structure of the hinge regions gives each of the 4 IgG classes their unique biological profile. Even though there is about 95% similarity between their Fc regions, the structure of the hinge regions are relatively different.